Singly coordinated silicon compounds

Carbon- and silicon-element single bonds in 4-coordinated carbon and silicon compounds. Polarities of the bonds. 

The single NMR resonance moved to low frequency with each addition of HMPA, and finally remained as a singlet at 8 = -78 ppm at all ratios of HMPA to silane of 3 1 or greater. The only reasonable structure for this new species is 5. These complex changes for a relatively simple system illustrate the subtle relationship between coordination and reactivity for silicon. It was observations such as these and others that stimulated us to try to make quantitative measures on hypervalent silicon compounds. 

Bond distances are influenced by coordination environment and, secondarily, by the nature of the substituents (electronegativity and size). A least-squares analysis of approximately two-hundred silicon compounds taken from the Cambridge structural data base has provided a consistent set of reference values for bonds between silicon and various nonmetals and these values are shown in Table 3, as well as the values expected for single bonds to silicon corrected for electronegativity differences. 

As demonstrated by single-crystal X-ray diffraction, the /-coordination polyhedra of 85-87 are distorted trigonal bipyramids, with each of the axial positions occupied by the oxygen atoms. This is shown for compound 86 in Fig. 11. In all cases, the crystals are formed from pairs of (A)- and (A)-enantiomers. Selected geometric parameters for 85-87 are listed in Table XIII. As can be seen from the Si-O [1.8004(10)-1.829(6) A], Si-N [1.741(7)-1.764(6) A], and Si-C distances [1.867(8)-1.915(2) A], the A/02N2C frameworks of 85-87 are built up by five normal covalent bonds and do not involve a bonding system in the sense of the 4+1 coordination usually observed for pentacoordinate silicon species with Si-N bonds. 

Most of the organosilicon compounds contain bonds between the silicon and carbon atom. In the following paragraph the structural chemistry of the Si—C single bond is discussed, mostly in compounds with tetracoordinate silicon and tetracoordinate carbon atoms. The structural chemistry of the Si—C bond in compounds where the carbon coordination state is different, is also discussed. The Si—C bond is markedly polarized and the increase of the bond ionicity by attaching different substituents to either the silicon or the carbon atoms may affect its length. The electronic and steric effects are discussed later. 

The coordination of donors lengthens the Si=N bond and leads to pyramidalization at the silicon centre, as can be seen from the deviation of the sum of angles around the silicon atom from 360° (see examples in Table 17). Coordination of silanimines to transition metal centres also increases the silicon-nitrogen bond length to 1.66-1.69 A, which corresponds to a Si—N single bond. It has already been mentioned above that these metal compounds are best described as metallacycles349,350. 

It is well recognised that, in some circumstances, the prior coordination of a nucleophile to tetracoordinate silicon can produce a pentacoordinate silicon centre that is activated to nucleophilic attack [1,8]. We have previously published preliminary data on the use of chelated pentacoordinate silylamides as reactivity probes for nucleophilic activation [8]. The work presented here is an extension of those studies and incorporates a stereochemical and kinetic investigation in a single series of experiments. The compound chosen for the experiments was 18 which has several advantages for a kinetic and stereochemical study. The coordinated A -methylimidazole (NMI) group is readily displaced by another NMI molecule in a degenerate substitution reaction. 

Recrystallization of this compound from water yielded the monohydrate 16-H20 (yield 81 %) which was studied by single-crystal X-ray diffraction [9], The coordination polyhedron around the silicon atom of 16 was fotmd to be a distorted trigonal bipyramid (Berry distortion [8] 18.7 %), with the carboxylate oxygen atoms in the axial positions (Fig. 5). 

In contrast to compounds 10-16 SiO C framework), the X Si-silicates 17 and 18 contain a S1O5 framework. The latter compounds were obtained by reaction of Si(OMe)4 with two mole equivalents of the respective a-hydroxycarboxylic acid and one mole equivalent of 2-(dimethylamino)ethanol in boiling acetonitrile (reaction time 50 h yield 61 % (17) or 77 % (18)). Recrystalli2ation of 18 from dimethylformamide yielded the 18 DMF solvate which was structurally characterized by single-crystal X-ray diffraction [11]. As can be seen from Fig. 6, the coordination polyhedron surrounding the silicon atom of 18 is a distorted trigonal bipyramid (Berry distortion [8] 13.9 %), in which the carboxylate oxygen atoms occupy the axial sites. 

Compound 28 was structurally characterized by single-crystal X-ray diffraction [5]. As can be seen from Fig. 12, the coordination polyhedron surrounding the silicon atom can be described as a distorted trigonal bipyramid (Berry distortion [8] 17.6 %), in which the oxygen atoms bound to nitrogen occupy the axial sites. 

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